SECTION 2. That in any proceeding for the enforcement of the provisions of the statutes relating to interstate commerce, whether such proceedings be instituted before the Interstate Commerce Commission or be begun originally in any circuit court of the United States, it shall be lawful to include as parties, in addition to the carrier, all persons interested, or affected by the rate, regulation, or practice under consideration, and inquiries, investigations, orders, and decrees may be made with reference to and against such additional parties in the same manner, to the same extent, and subject to the same prov ions as are or shall be authorized by law with respect to carriers.